1. Field of the Invention
This invention generally relates to materials for converting microwave energy into thermal energy, and further to microwave cooking receptacles which brown and sear food by using materials of this type. In particular, this invention relates to a compound made up of a high impedance soft magnetic material and other materials, and which has improved microwave conversion properties and lower cost, and to cookware made from the compound. Examples of suitable high impedance soft magnetic materials include ferrites and soft magnetic iron powder. Although the specific example given below uses iron oxide, those skilled in the art will appreciate that iron powder could be substituted for the purpose of even further lowering the cost of the material.
2. Description of the Related Art
Compounds of iron or iron oxide have been known since the beginning of the "iron age." More recently, it has been proposed to use a ferrite coating on a disposable container to convert microwave energy to heat in order to improve the browning, crisping, or searing properties of the container. Such a coating is described, for example, in U.S. Pat. No. 5,079,398. While a heating effect is said to be achieved (370.degree.-400.degree. F. in less than about 5 minutes or 120.degree.-140.degree. F. within about three minutes), however, the time it taken to reach an adequate temperature is unacceptably slow and the coatings described therein are costly and difficult to manufacture and also relatively costly in comparison with the material and cookware structure of the present invention.
Both the present invention and the material described in U.S. Pat. No. 5,079,398 are directed to the same problem, namely that certain foods cooked in a microwave oven are, by their appearance, displeasing because the microwave cooking process does not produce a color and texture in the food similar to that produced by conventional cooking methods, i.e., microwave-cooked food often is not seared, browned, and crisp, and in addition the compound of the present invention can use ferrite as one of the elements of the compound, the similarities essentially end there. The present invention provides a much more practical solution to the problem than did prior approaches, with better results such as improved durability and a lower heating time.
Although the present invention involves material which directly converts microwaves to thermal energy, those skilled in the art will appreciate by way of background that an alternative type of non-iron oxide based converting material can be fabricated from a plurality of resistive particles, each of which functions as an independent electrode so that adjacent pairs of particles form spark gaps. This alternative type of material is less relevant to the present invention than the direct conversion materials, and has the separate disadvantage that, when the resistive particles are irradiated with microwave energy, electric arcs form across the spark gaps, thereby producing an amount of thermal energy sufficient to sear and brown food. Converting materials of this type are disclosed, for example, in U.S. Pat. Nos. 4,496,815, 3,777,099, 3,731,037, and 3,701,872. In use, this type of converting material has at least the drawback that, in order for it to reach a temperature high enough for cooking, it has to be preheated in a microwave oven. This preheating step adds to the overall cooking time of the food, and thus undercuts what is generally accepted to be the primary advantage of microwave cooking--rapid food preparation.
Other types of converting materials such as semiconductor films do not require preheating (see, for example, U.S. Pat. Nos. 5,239,153, 4,970,360, 4,948,932, and 4,641,005), but such materials are in general primarily used in disposable cooking containers, and are not well suited for use in permanent cookware which requires washing and re-use on a daily basis.
It is clear from the foregoing discussion that converting materials presently in use in microwave cookware have drawbacks which limit, either in terms of convenience or economics, the everyday life of the consumer. A need therefore exists for an improved converting material which, when incorporated into the cooking surface of a microwave oven receptacle, will brown and sear food without realizing any of the disadvantages of existing microwave cookware.